Heretofore for the purpose of protecting electronic devices and electric devices against damage due to overvoltage, there have been utilized nonlinear voltage resistors using silicon carbide (SiC), selenium (Se), and silicon (Si). In recent years, a nonlinear voltage resistor (hereinafter referred to as a "ZnO varistor") has made of a sintered article produced by admixing ZnO as a main component with various additives, molding the resultant mixture into a prescribed shape, and sintering the molded mixture. The ZnO varistor has characteristics such as a low discharge voltage and a large nonlinear voltage coefficient. It is, therefore, useful for protecting against overvoltage devices made up of semiconductor elements that have small capacities for resistance to overvoltage. The popularity of the ZnO varistor has been growing as a replacement for a SiC varistor and other overvoltage protection devices.
Among the types of ZnO varistors so far adopted for actual use is counted the ZnO-Pr.sub.6 O.sub.11 type. It is known that the ZnO-Pr.sub.6 O.sub.11 type varistor is produced by admixing ZnO as a main component with cobalt (Co), magnesium (Mg), calcium (Ca), potassium (K), and chromium (Cr) beside Pr as auxiliary components used in the form of elements or compounds and firing the resultant mixture. This is described in Japanese Patent Publication No. SHO 57(1982)-42,962.
The ZnO varistor is well known as a semiconductor ceramic and has electrical properties that are notably altered by the presence of a minute amount of impurities. When a monovalent metal such as sodium (Na) or lithium (Li) is added, the ZnO varistor increases its magnitude of resistance because the added metal functions as an acceptor. When a trivalent metal such as aluminum (Al) or iron (Fe) is added, the ZnO varistor loses its magnitude of resistance because the added metal functions as a donor. As demonstrated in Japanese Patent Publication No. SHO 55(1980)-37,846, Al is known to be an element capable of improving the discharge voltage property at a very low addition rate. Among the trivalent metals, Fe is a problematic element because this element, even at a very low addition rate, increases leakage current and deteriorates the discharge voltage property.
Since the ZnO varistor has its properties altered by impurities as described above, the compound ZnO used as the raw material therefore must be highly pure. Ordinary grade ZnO powder used as a white pigment is not acceptable.
The ZnO powder that is used for the ZnO varistor is mostly produced by the "France Method" that comprised fusing metallic Zn in a kettle made of carbon, for example, and oxidizing the Zn vapor issuing from the molten Zn with air. By this method, ZnO powder of extremely high purity can be produced. The kettle for the fusion of the metallic Zn is used for a long time and, therefore it produces a sediment of impurities. This sediment contains Fe in a large proportion. As a result, the proportion of Fe sediment that passes into the Zn vapor increases with the increasing number of days of use of the kettle. The Fe content of the produced ZnO powder, therefore, gradually increased with the increasing number of days of use of the kettle from an initial level of 2 atomic ppm to a level about 15 atomic ppm.
The varying Fe content of the ZnO powder from one lot to another of the raw material increased the possibility that the V-I property of the ZnO varistor will vary from one lot to another of the ZnO raw material. The conventional method has not always been capable of providing elements with stable properties.